This invention is related to a method of making certain kinds of tubing. It more specifically refers to a method of making a multiple walled tube, especially a two concentric walled tube with the two walls being radially spaced apart and having rib members in supporting relationship between and attached to the walls. The rib members of the composite tubular structure of this invention have been referred to as ribs, webs, struts and other identifying names. All of these members are intended to be included in this invention regardless of the name by which they are called. For ease of description and understanding, this invention will be described in relation to a two concentric walled tube. It will be understood that tubes with more than two concentric walls are adapted to be made according to this invention. Further, although the instant description generally refers to hollow, multi-walled tubes, it should be clear that multi-wall tubes with a solid inner element can be made according to this invention. Still further, it should be clear that the manufacturing method and apparatus described herein could be used to make a multi-walled, hollow core tube with an armature residing in the hollow inner tube. The armature can be rigid or flexible and may be made of a different material than the multi-walled tube. It can be wholly or partially in contact with the interior wall of the inner tube.
Core members are widely used in the plastic film industry as well as in the carpet and other textile fabric industries. Rolls of material are wound around suitable cores for shipping and as a convenient form for the material rolled thereon to be used as feed to downstream operations for processing the material rolled onto the core. It is common for these large rolls of material to be wound quite tightly because if the volume of the rolled material is minimized, shipping costs are also minimized. It is also a fact that as material is wound on a core, it is under at least some tension. Therefore, when the roll is completed, the rolled material tends to collapse upon itself thereby exerting more or less crushing force against the core. In addition, shrink or stretch wrap film is also conventionally wound on cores. This type of film was made with at least a unidirectional stretching operation and it therefore has a strong tendency to shrink, that is, the film has a memory wherefore its length (and sometimes its width as well) always tries to shorten itself. This tendency causes an even greater crushing stress on the core around which the shrink wrap film is wound.
It has been conventional to use disposable cardboard core members. However, it has been found that, although cardboard is very inexpensive and that is a very strong recommendation for its use, it does not have a crushing strength that is sufficient for it to withstand higher inwardly directed forces that are inherent in some wound materials, especially wound plastic film.
In order to rectify this situation, the industry has used metal (hollow or solid), wooden and/or solid plastic cores. Some thought has been given to using hollow plastic cores but it has been found that the cores must be exceptionally thick so as to be able to withstand the inwardly directed radial pressure that the wound material exerts on the core. Therefore, so much plastic must be used to make a core of the desired crush resistance, that it is uneconomical.
In the prior art, core elements have been used that consist of a solid material, such as in the form of a solid cylinder. Alternatively, cores are known that are in the form of a hollow cylinder having a single, relatively thick wall. Cores of this configuration, for example made of metal or plastic materials, can have excellent crushing strengths. The disadvantage of such cores is that they are generally quite expensive. If the user returned the cores after use, the initial expense of purchasing the cores would not be such a detriment because the shipper/seller could then reuse the returned cores. The cost of the cores would then be amortized across multiple uses rather than a single disposable use. However, users have not generally returned the used cores. The manufacturer can only amortize the cost of the core against a single use and thus must add the cost of the cores into his selling price of the material that is wound about the core. This substantially increases the price at which the wound material must be sold whereby making it less competitive. All in all, these prior art plastic cores have not met with any substantial degree of success and there is need for improvement in this technology.
Recently, a modified tubular core structure has been invented that has sufficient crushing strength to be useful in industrial applications, and employs little enough material to be economically viable. This tubular core is sufficiently inexpensive so that it does not add a significant amount to the cost of the material wound on the core wherefore it does not have to be returned to the seller. This core material is made up of concentric tubular members (preferably 2) with rib members disposed there between and in supporting relationship to both tubular members. The interior element of this core structure can be solid or hollow. Tubes with more than two concentric elements are contemplated.
Other uses have been found for this novel tubular structure in an internally hollow configuration. These structures find application in many industries such as: agriculture, construction, irrigation, water and sewer distribution, telecommunications and other electrical conduit markets. While smaller diameter multi-walled tubes have use as winding core elements, larger diameter multi-walled, hollow, tubular structures are useful in drain pipe and culvert applications. A further aspect of this invention, then is the manufacture of larger diameter, multi-walled, hollow, composite structures that are both relatively light in weight and strong in flex and crush strengths.
It is interesting to note that the material from which the tubular structure of this invention is made can be varied depending on the use to which the structure is to be put. For use as a core upon which sheet-like materials are to be wrapped, the composite tubular member should be relatively stiff and not either flexible or crushable. For culvert applications, the tubular structure should have excellent crushing strength but also may have sufficient longitudinal flexibility to be able to be bent around structures that are encountered in the ground, such as large boulders. For drain pipe and septic system applications, the tubular structure can be quite flexible, although generally, the flexibility of the pipes of the instant invention is somewhat less than conventional corrugated pipe. Further, for drain pipe and septic field applications, the structures of this invention can be perforated. The multi-walled tubular product of this invention has unique application in its perforated form. Ground water draining through the outer wall will be directed into channels (preferably helical channels, that exist between the inner and outer walls and then will drain to the end of the tube without necessarily penetrating through the inner wall and into the interior hollow structure. This structure enables the inner tubular element to be solid and therefore much more supportable. Of course, it is considered to be an embodiment of this invention to utilize the products made by the manufacturing process and apparatus of this invention as drain pipes with perforations through both the inner and the outer tubes.
This tubular structure to which this invention is directed comprises a plurality of elongated tubes, preferably of extrudable material such as plastic (e.g. polystyrene, polyethylene or the like) or aluminum or other metals. The tube is made up of multiple concentric tubular members that are radially spaced from each other. It is anticipated that the concentric tubes will be coextruded.
At least one, but preferably a plurality, of radially directed rib(s) is disposed between and in supporting relationship to the concentric tubular members. The rib(s) serves to maintain the relative radial positions of the concentric tubular members, and to provide substantial crushing resistance without adding significant amount of extra material. In many applications, the rib will be substantially normal to the tangent to the interior and exterior tubular members. Although struts that are disposed normal to the surfaces of the concentric tubes are suited to this use, it has been found that even greater strength increase and weight minimization are achievable if the ribs are disposed at an angle, other than right, with respect to the tangents of the interior and exterior tubular elements so as to form generally triangular rib members.
Next adjacent ones of these angularly disposed supporting rib elements are suitably in contact with each other at one end thereof. Thus, the two ends of at least some, but preferably all, of the angularly disposed supporting rib members are in supporting contact with the outside surface of the inner tube, and the inside surface of the outer tube, respectively. At, or very near, the line along which the angular supporting rib member is in contact with either the inner or the outer tube, each angular supporting member is also preferably in contact with the next adjacent angularly disposed supporting rib member. Thus, if the two adjacent angularly disposed supporting rib members are in contact with each other, they, together with the tubular wall that is opposite to their contact point, form a generally triangular cross section longitudinal rib member.
If the next adjacent angularly disposed supporting rib members are spaced a small distance apart at the location where they contact the respective outer and inner walls of the concentric tubular members, respectively, the assembly of two next adjacent support members and two opposite segments of the concentric tubular wall elements form a truss assembly that has a cell or cavity that is generally trapezoidal in cross section. It has been found that if the length of the shorter wall segment of one of the tubular elements is relatively short, for some unexplained reason, composite tubes with rib members having such a trapezoidal cross section are stronger than the those with triangularly shaped rib members. In either case, it is preferred, but not absolutely required, that the rib members extend the entire length of the tubular article. These rib members should preferably have a circumferential as well as a longitudinal component to their direction.
It has also been found preferable to dispose a plurality of rib members about (preferably evenly about) the entire cylindrical space between the inner and outer tubular members. Suitably, the disposition of the rib members should be symmetric about the outer and inner, respectively, circumferences of the tubular members. It has been found to be most preferred to have a plurality of rib members disposed throughout the entire cylindrical space between the inner and the outer tubular members with each adjacent rib member sharing a wall with its next adjacent rib member.
In the preferred embodiment of this structure, the rib members are disposed in a longitudinally helical configuration. This structure imparts excellent crush resistance because it provides a component of stiffness that is both tangential and radial to the walls, and it also provides the lightness that is characteristics to this product. Further, in appropriate situations, and with the correctly designated construction material, the helical configuration of the rib members adds a significant amount of longitudinal and radial bending stiffness. The angle that the helix makes to the longitudinal axis of the instant product is a determinant of the amount of bending stress can be withstood (that is how radially stiff the product is).
An interestingly adjunct of this structure is the fact that the process of forming the ribs/trusses into helical configuration unexpectedly increases the degree of roundness of the final product. Thus, the formation of this helical rib configuration causes the cross section of the composite tubular product to remain substantially constant and symmetrical. That is, if the concentric tubular members each have a circular cross section, the helical configuration of the rib members tends to cause the entire article to have a circular cross section and minimizes distortion of the circular cross section into an elliptical or oblate cross section, or a cross section with hills and valleys.
It will be appreciated that making hollow tubes of this configuration on a commercial scale is a very difficult undertaking. The instant invention is directed to a particularly effective method of making these articles and apparatus suited for carrying out this method.